This invention relates to fuel cell systems and, in particular, to multi-stack fuel cell assemblies.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. Molten carbonate fuel cells (“MCFC”) systems operate by passing a reactant fuel gas through the anode, while oxidizing gas is passed through the cathode.
In order to produce a useful power level, a number of individual fuel cells are conventionally stacked one on the other to form a fuel cell stack. The number of cells in the stack determines the power rating of the stack and to provide systems with higher power ratings, a number of fuel cell stacks are utilized and the outputs of the fuel cell stacks are combined to provide the desired power output. An example of a multi-stack fuel cell system is described in U.S. application Ser. No. 10/916,235, assigned to the same assignee herein.
The fuel cell stacks produce electrical output in the form of a direct current (DC). The DC current produced by the fuel cell stack can be converted to conventional AC power using an inverter. In a single-stack assembly, the fuel cell stack is coupled to an inverter and DC current produced by the stack is drawn by the inverter to convert the DC current to AC power. Conventional multi-stack assemblies employ a DC bus to connect the positive polarities of the stacks and the negative polarities of the stacks in the assembly so that the multi-stack assembly functions as a single power source. The DC bus has a plurality of inverters coupled thereto and the DC current supplied to the DC bus is drawn by the inverters and converted to AC power. An example of this multi-stack assembly is shown in FIG. 1, in which the assembly 1 includes four fuel cell stacks 2a-2d, a DC bus 3 to which the positive polarities and the negative polarities of the stacks 2a-2d are connected, and three DC-to-AC inverters (4a-4c) connected to the DC bus and drawing DC power from the DC bus to convert the DC power to AC power. AC power produced in each inverter 4a-4c is supplied to a corresponding transformer 5a-c, which steps up the voltage of the inverter output to 480V. AC power at 480V is supplied to a customer bus 6 and thereafter to the consumer 7.
The advantages of conventional assemblies, as shown in FIG. 1, are that the number of inverters required is fewer than the number of the stacks in the assembly and that the assembly is capable of continued operation if one of the inverters fails. However, the assembly is not capable of continued operation if one of the fuel cell stacks in the multi-stack assembly fails since there is no means for isolating the failed stack from the fuel and oxidant gases supplied to the other stacks in the assembly. The use of the DC bus in conventional multi-stack assemblies does not allow biasing of individual stack currents through control of individual inverters so as to reduce the amount of DC current drawn by the inverters if one of the stacks in the conventional multi-stack assembly is weak and produces less DC power than the other stacks. Moreover, the use of the DC bus requires customization of the DC bus, which results in increased manufacturing and operating costs.
Accordingly, a multi-stack assembly having a simple and cost effective construction while avoiding the above disadvantages is needed.
It is therefore an object of the present invention to provide a multi-stack assembly that is cost effective and has a simple construction without requiring a DC bus and which is capable of continued operation if one of the fuel cell stacks in the assembly fails or produces less power than the other stacks.
It is a further object of the present invention to provide a multi-stack assembly which allows for increased power output without affecting fuel cell durability and without requiring additional electrical isolation.
It is a further object of the present invention to provide a method of controlling a multi-stack assembly which allows control of individual inverters in the assembly.